Vapor-phase polymerization of a polymerizable monomer or mixture thereof to produce normally solid polymer substances using a quench-cooled, vapor-phase polymerization reactor containing a subfluidized particulate bed of polymerized monomer has been described in a number of patents including: U.S. Pat. No. 3,957,448 (Shepard et al.), U.S. Pat. No. 3,965,083 (Jezl et al.) and U.S. Pat. No. 3,971,768 (Peters et al.), the disclosures of which are specifically incorporated herein in their entirety by reference. These U.S. Patents, assigned to the assignee of the present invention, describe polymerization processes and apparatus in which polymer is formed from gaseous monomer in horizontal stirred-bed vessels.
In a single reactor, polymerization of monomer or mixture thereof from the vapor state is carried out by an essentially isobaric process typically using a high yield catalyst and cocatalyst. Typically, in operation of such processes and apparatus, particles of polymer are formed around solid catalyst particles.
The horizontally disposed reactor vessel has recycle propylene gas introduced into the bottom thereof together with hydrogen gas. Typically, quench liquid, such as liquid propylene, is injected into the reactor from the top of the reactor. The hydrogen is provided for molecular weight control.
Gases and vapors within the reactor vessel are free to circulate and mix together throughout the vapor space. For continuous production of some polymers, particularly copolymers, where it may be necessary to have different gas compositions at subsequent stages of polymerization, a series of two or more reactors is required.
Paddle wheels or other types of stirring vanes inside the vessel sweep through the bed of polymer particles and stir the contents of the vessel. The various types of stirring vanes including staggered paddles, inclined paddles, spiral vanes, or vanes provided with a scraper for scraping the internal wall of the reactor vessel.
Near one end (front end disposed opposite to a take-off end) of the horizontal vessel a catalyst system comprising a catalyst injected at least one point into the top of the vessel, and a cocatalyst plus modifier injected at a point adjacent the point of injection of the catalyst, is injected into the top of the vessel.
Solid particles of polymerized monomer are created in the vessel and are withdrawn from the take-off end thereof. Particles of polymerized monomer build up in the stirred reactor and traverse the length of the reactor essentially because of polymerization in the bed and not by the agitator. Advantageously, this condition is ensured by the design of the agitator such as to provide for agitation, but not for backward or forward movement of the particles. Since a stirred bed is not in a fluidized condition, back-mixing of the particles of polymerized monomer in the horizontally disposed reactor vessel is limited. In contrast, solid particles in a fluidized bed are very well mixed. Even at commercially useful ratios of length to diameter, horizontal stirred-bed reactor systems can readily achieve a degree of mixing of solids equivalent to two, three, or more theoretical back-mix reactors. Thus, horizontal stirred-bed reactor systems are particularly advantageous, as compared fluidized-bed reactors, for direct production of polymers in a particulate form.
It is desirable to create polymer particles as quickly as possible, and for this purpose a number of different high activity catalyst systems have been developed.
Use of solid, transition metal-based, olefin polymerization catalyst components is well known in the art including such solid components supported on a metal oxide, halide or other salt such as widely-described magnesium-containing, titanium halide-based catalyst components. Such catalyst components commonly are referred to as "supported."
As is well known in the art, particulate polymers and copolymers may be sticky, i.e., tend to agglomerate, due to their chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers also are referred to as non-free flowing polymers because of their tendency to compact into aggregates of much large size than the original particles and not flow out of the relatively small openings in the bottom of product discharge tanks or purge bins. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor, however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the aggregates which form and the bed will not refluidize.
Sticky polymers also can be defined by their flow, called the Flow Factor, which references the flow of all materials to that of dry sand. On a scale of 1 to 10, the Flow Factor of dry sand is 10. The Flow Factor of free flowing polymers is about 4 to 10 while the Flow Factor of non-free flowing or sticky polymers is about 1 to 3.
Means for powder transfer between vertically disposed fluidized bed reactors is described with apparatus for gas phase polymerization of alpha-olefin in U.S. Pat. No. 4,703,094 and U.S. Pat. No. 4,902,483. The powder transfer means described includes three serially connected vessels (a discharge vessel, a decompression vessel and a compression vessel) and a pneumatic lifting system which uses reaction gas from the downstream reactor. The patents teach that times of contact of the powder with reaction gas from the downstream reactor in the compression vessel and the pneumatic lifting system must be very limited: less than or equal to 60 seconds in the compression stages, and less than or equal to 180 seconds in the pneumatic lifting stage. Because reaction gas from the downstream reactor is used in the compression vessel and the pneumatic conveyance of the powder, reaction gas from the downstream reactor is transferred into the upstream reactor with gas recycled from the decompression vessel into the upstream reactor.
Although polymers that are sticky can be produced in non-gas phase processes, there are certain difficulties associated with the production of such products in, for example, slurry or bulk monomer polymerization processes. In such processes, the diluent or solvent is present in the resins exiting the reaction system at a high concentration leading to severe resin purging problems particularly if the material in question is a low molecular weight resin or a very low crystallinity resin. Environmental considerations are such that the dissolved monomers and diluent must be removed from the polymer prior to its exposure to air. Safety also dictates the removal of residual hydrocarbons so that closed containers containing the polymers will not exceed safe volatiles levels in the gas head space over the resin. The safety and environmental concerns are accompanied by a definite economic factor in determining a preference for a quench-cooled, vapor-phase polymerization reactor containing a subfluidized particulate bed of polymerized monomer. The low number of moving parts and the relative lack of complexity in a basic subfluidized bed process enhances the operability of the process and typically results in lower costs of production. Low costs of production are due, in part, to low volumes of recycled process streams and a high unit throughput.
Horizontal stirred-bed reactor systems disclosed in Shepard et al., Jezl et al., Peters et al., and in U.S. Pat. No. 4,101,289 ('289), U.S. Pat. No. 4,129,701 ('701), U.S. Pat. No. 4,535,134 (de Lorenzo et al.), U.S. Pat. No. 4,627,735 (Rose et al.), U.S. Pat. No. 4,640,963 (Kreider et al.), U.S. Pat. No. 4,883,847 (Leung et al.), U.S. Pat. No. 4,921,919, (Lin et al.) and U.S. Pat. No. 5,504,166 (Buchelli et al.), the disclosures of which are specifically incorporated herein in their entirety by reference, largely or completely solve problems related to vapor phase, solution or slurry polymerization and reaps important economic benefits through savings in energy consumption, raw materials, and capital costs.
Although previously known vapor-phase polymerization systems are entirely satisfactory for manufacture of many commercial polymers, a need still exists for an improved method and/or apparatus for transfer of growing polymer particles between high pressure, reactive gas-filled, continuous, vapor-phase polymerization reactors while maintaining each at independently selected operating conditions. Desirably, the improved process produces fewer fine particles (fines) during transfer. Such fines tend to hang-up or become trapped in transfer equipment and can even plug lines and valves. More desirably, the improved transfer apparatus increases the range in physical properties of polymers which can be manufactured at high rates of production without interruptions in operation. Especially welcome are improved methods and/or apparatus which more closely achieve continuous steady-state conditions throughout the vapor-phase process and thereby produce polymer products having more uniform physical properties.
One problem with known polymerization processes and apparatus using a vapor-phase polymerization system having two or more serially disposed polymerization reactor vessels, is reliable and accurate methods for interreactor transfer and metering of polymer particles from upstream to downstream, high pressure, reactive gas-filled, continuous, vapor-phase polymerization reactors while maintaining each reactor at independently selected operating conditions.